The preparation of ascorbyl fatty acid esters chemically only takes place under extreme conditions, for example in sulfuric acid or in hydrogen fluoride, see, for example, DE 27 43 526 A1, DE 33 08 922 A1 or DE 28 54 353 C3. Processes of this type are questionable in their industrial implementation. The products need laborious purification. Products which are prepared by these processes have an odor which is not accepted by the user even after customary working up.
EP 0 401 704 B1 describes a process for the preparation of an organic ester of ascorbic acid comprising the reaction of ascorbic acid in the presence of 100 to 10,000 ppm of water and of an organic acid or an ester thereof in an organic solvent in the presence of an ester hydrolase. The yields reported here in Tables 1 to 4, however, do not exceed the value of 2%, so that this process has not found its way into industry.
C. Humeau et al., Biotechnology Letters, Vol. 17, No. 10, pp. 1091 to 1094 describe the synthesis of ascorbyl palmitate in a nonaqueous medium using an immobilized lipase, obtained from Candida antarctica, as a biocatalyst. The enzymatic synthesis is described as perfectly regioselective. It is stated that when using methyl palmitate as the acyl donor 68% of the ascorbic acid is converted. These values were obtained with an initial quantitative ratio of ascorbic acid to acyl donor of 1 mol to 5 mol and a very low use concentration. Reworking of the appropriate experiment, however, resulted in far lower reaction yields, so that this process is also not suitable for industrial preparation of the fatty acid esters of ascorbic acid.
In the papers of the meeting of the 1st workshop "Bioconversion of Renewable Raw Materials", Jun. 30th/Jul. 1st 1997 Detmold, pp. 57 and 58, enzymatic syntheses of sugar esters were presented by B. Haase et al. It is shown that it is increasingly possible by selective, enzyme (lipase)-catalysed esterifications to obtain better defined, even regioisomerically pure products. As an example of a sugar ester, a 6-O-acyl-L-ascorbic acid is mentioned without indication of an actual preparation process.
JP-A-4-141093; Mitsubishi Rayon Co. Ltd. describes the esterification of fatty acids with ascorbic acid using an enzymatic catalyst in a solvent which forms an azeotrope with water. The yield is only 37% after a reaction time of 72 h or 18% after the same reaction time if the water of reaction is not removed during the reaction. Moreover, the process necessitates the control of the water content in the reaction mixture in a concentration range from 200 to 2000 ppm.